Many electronic devices use a multi-output power supply unit (PSU) to provide power to a number of circuits and electrical components. The power supplied should be regulated to insure that the components operate correctly and are not damaged by an overload of excessive volts. If one output of a PSU is connected to a varying load, the output typically should be regulated to prevent significant voltage changes. If a PSU has one well regulated output connected to a varying load, the voltages seen at the other outputs may often rise significantly. As an example, if a regulated 5 volt output is heavily loaded, a semi-regulated output ordinarily at 12 volts may rise to 24 volts or more, representing 200%+ of the nominal voltage. This voltage difference may damage certain electrical components and even the entire device connected to the output. For example, if a hard drive in a personal computer is powered by a 12 volt output rail from a PSU and the hard drive is switched to standby mode, the load on that rail will generally decrease. This decreases the current drawn by the load, thereby causing the rail's output voltage to rise. Should the output voltage increase to more than 10% above nominal or even double to 24 volts, it may overload some components of the hard drive and the hard drive may fail.
There are several options currently available to more accurately regulate the output voltage of a multi-output power supply. One is to add a post regulator to the semi-regulated output. However, this type of regulator often takes up a large amount of space on a circuit board, adds complexity to the circuit, may be inefficient as it generates a significant amount of heat and can be expensive to implement. Another option is to add a pre-load resistor to the semi-regulated output. This option can be relatively inefficient as it also generates a substantial amount of heat, which wastes energy and raises the overall temperature of the circuit. A third option is to implement a more expensive transformer as part of the PSU. This is often more efficient than the first two options, but may not be feasible for low-cost electronics as it significantly raises the cost of the PSU circuit. Still another option is to add an electronic magnetic regulator to the semi-regulated output. However, the regulator may not work over a wide load range as it has a slow response to sag and surge conditions and is sensitive to a lagging power load factor.